Composite structural elements and method of making same

ABSTRACT

A composite structural element and a method for making same are provided. The element includes a polymer foam core and at least one fibrous layer adhered to the polymer foam core by epoxy. Nano-particles are suspended in the epoxy prior to curing; preferably they are mixed with the hardener before it is mixed with the resin. The polymer foam core is preferably an exothermic foam such as polyurethane, and heat generated by the exothermic foam cures the epoxy, thereby causing the fibrous layer to adhere to the foam core. The nano-particles may be made from at least one of carbon, a ceramic, tungsten, a carbide, titanium, zircon, aluminum, silver, or boron. When carbon nano-particles are used, the strength of the composite is greatly increased, and the curing time of the heat-curable epoxy is significantly reduced. Ceramic nano-particles can be used to increase penetration resistance and provide increased ballistic protection.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to structural elements used in construction andinfrastructure, and more specifically to composite structural elementshaving a fibrous layer impregnated with epoxy, preferably adhered to afoam core.

2. Description of Related Art

Traditional composites gain much of their strength by the type of fibers(carbon fiber, Kevlar, fiberglass, etc.) that are included in thecomposite structure. For many composite applications, such as buildingmaterials, these fibers prove to be too costly and limit their use.

Due to cost constraints, there are no composite-based structuralinsulated panels on the market. Traditional structural insulated panelsare made from either wood or steel structures that are insulated byeither polyurethane or polystyrene. The insulating material is“sandwiched” between two layers of the wood or steel outer skin. Bondingbetween layers is done with various glues.

These traditional composites use expensive fiber materials such as thosementioned above to achieve high strength. This limits their use incost-sensitive industries such as building and infrastructure. Otherpanel systems used for walls, roofs and floors are comparable in costbut have inherent material issues such as rotting, corrosion, anddelamination between the insulating core and the exterior skins. Inaddition, other panel systems require additional layers to be added totheir exterior before finishing the wall with stucco or mud.

One excellent composite-making method is described in U.S. Pat. No.6,117,376 to Merkel, the instant inventor, and which is assigned to theinstant assignee; the teachings of U.S. Pat. No. 6,117,376 areincorporated by reference herein. In this method, a mold is created andcarbon fiber layers or skins line the inside surfaces of the moldhalves. The carbon fiber layers are wetted with heat-curable epoxy, andthe mold halves are closed together to form an interior cavity having anopening. Exothermic foam is introduced into the mold cavity in an amountso as to overfill the mold. The foaming material expands and heats up,causing the heat-curable epoxy to cure. Since an abundance of foam isprovided, excess foam spills out of the opening in the cavity, butbecause the foam expands so rapidly, the pressure inside the mold causesthe carbon fiber layers to be pressed flat against the inner surfaces ofthe mold. In addition, the epoxy cures (from the heat generated by theexothermic reaction of the foam) and causes the carbon fiber layers tostick securely to the internal foam. The resulting composite isexceptionally strong and can be created in substantially any shape.However, while the method of manufacture is easy and inexpensive and caneven be performed on the construction site, as mentioned above, thecarbon fiber layers are too expensive to make the resulting product apractical building material.

In addition to new construction, traditional infrastructure repair islacking for similar reasons. For example, in repairing a columnarsupport of a bridge substructure, the conventional approach is to weldpre-rolled steel panels to the exterior of the column, and then backfillthe structure with concrete. The problems with this approach aremanifold. For one, the steel and concrete are extremely heavy and thushave high shipping costs. In addition, in performing this retrofit, thefooting of the column must be replaced, and the roadway above must beclosed. Finally, the process takes an unacceptably long time tocomplete.

Accordingly, there is a long-felt need for a building structure andmaterial that is light weight, easy to manufacture, and cost-efficientas well, that can be used for both new construction as well as forrepair purposes.

SUMMARY OF THE INVENTION

The invention is a composite structural element and method of makingsame. In one embodiment, the invention is a composite structural elementhaving a polymer foam core and at least one fibrous layer adhered to thepolymer foam core by epoxy, preferably heat-curable epoxy.Nano-particles are suspended in the epoxy while the epoxy is still inliquid form. Preferably, the polymer foam core is an exothermic foam,more preferably a polyurethane, wherein heat generated by the exothermicfoam cures the epoxy, thereby causing the fibrous layer to be adhered tothe foam core. The composite structural element preferably also includesa bulk laminate layer disposed between the polymer foam core and thefibrous layer.

In one embodiment, the nano-particles include carbon nano-particles,preferably having a diameter of approximately 10-200 nm. In anotherembodiment, the carbon nano-particles include nano-fibers approximately60-200 nm in diameter and approximately 30-100 μm in length. In eithercase, the weight ratio of the carbon nano-particles to the epoxy ispreferably at least 1:200 and no more than 1:2. More preferably, theweight ratio of the carbon nano-particles to the epoxy is at least 1:100and no more than 1:10. In one embodiment, a weight ratio of the carbonnano-particles to the epoxy is substantially 1:10, and the structuralelement is a thermally conductive roofing panel. The nano-particlescould also include at least one of carbon, a ceramic, tungsten, acarbide, titanium, zircon, aluminum, silver, or boron. Preferably, thenano-particles having a high thermal conductivity relative to the epoxy,so that the addition of the nano-particles reduces the curing time ofthe epoxy.

The fibrous layer of the composite preferably includes at least one of awoven polyester mat and/or a fiberglass mat. The fiberglass mat includesat least one of a chopped-strand fiberglass mat or a continuous filamentfiberglass mat. The fibrous layer may include two fiberglass mats, eachof the fiberglass mats being either a chopped-strand fiberglass mat or acontinuous filament fiberglass mat.

In a preferred embodiment, the composite structural element of theinvention is a structural insulated panel. In this embodiment, the atleast one fibrous layer further comprising two fibrous layers onopposite sides of the foam core, both of the two fibrous layers adheredto the foam core by the nano-particle-impregnated epoxy. Optionally, thestructural insulated panel may include an integrated dry wall finishapplied to a first outer surface of one of the two fibrous layers,and/or it may include exterior masonry applied to a second outer surfaceof the other of the two fibrous layers.

In another embodiment, the nano-particles include at least one ceramic,preferably boron carbide, and the structural element provides ballisticshielding protection.

In another embodiment, the nano-particles are carbon and the structuralelement is a structural wrap formed around an existing load-bearingelement. More specifically, the structural wrap may include a bridgesubstructure wrap and the existing load-bearing element is a bridgesupport column. In this embodiment, the wrap substantially encompassesall of the bridge support column; the foam core extends along at leastapproximately 80% of the length of the column.

The invention also includes a method of making a composite structuralelement. In the inventive method, nano-particles are first mixed into anepoxy, preferably a heat-curable epoxy. At least one fibrous layer isprovided, and the fibrous layer is wetted with thenano-particle-impregnated epoxy. A heat source is then introduced tocure the epoxy. Preferably, the heat-introducing step entails providinga catalyzed foamable exothermic material in thermal communication withthe wetted fibrous layer. In one embodiment, two wettednano-particle-impregnated fibrous layers are provided in a mold, and theexothermic foam is introduced in direct contact with and in between thefibrous layers, thereby curing the epoxy and bonding the fibrous layersto the foam to form a foam core sandwiched between the two fibrouslayers. Alternatively, a flexible receptacle having a wall, at least oneopening, and an interior may be placed between the two fibrous layers,and the exothermic foam is introduced into the interior of thereceptacle via the opening. The epoxy is thus cured with heat releasedfrom the exothermic foam that passes though the wall of the receptacle.

In adding the nano-particles to the epoxy, the mixing step preferablyincludes the steps of: adding the nano-particles to the hardener of theepoxy, and then mixing the nano-particle-impregnated hardener with aresin. The nano-particle mixing step is preferably performed so that aweight ratio of the nano-particles to the epoxy is at least 1:200 and nomore than 1:2, and more preferably at least 1:100 and no more than 1:10when using carbon nano-particles. However, the nano-particles addedduring the nano-particle mixing step may include at least one of carbon,a ceramic, tungsten, a carbide, titanium, zircon, aluminum, silver, orboron.

In another embodiment of the inventive method, the wettednano-particle-impregnated fibrous layer is placed before curing(preferably in situ) around an existing load-bearing element. A mold anda second wetted nano-particle-impregnated fibrous layer are placedaround the first fibrous layer and spaced apart from the first fibrouslayer. The exothermic foam is then introduced into the mold between thetwo wetted fibrous layers. In this way, an existing load-bearingelement, such as a bridge support column, may be rapidly reinforced orrepaired without taking the column out of service. In these type ofsituations, the foam is provided along at least approximately 80% of thelength of the existing load-bearing element.

The present invention offers a method whereby relatively inexpensivefibers can be used by including nano-particles into the epoxy-compositematrix. The resulting composite structure nears the strength andperformance properties of traditional composite structure, whilereducing the cost of the finished product.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side sectional drawing of a composite structural insulatedpanel made in accordance with the invention.

FIG. 2 is a broken perspective drawing of a composite structuralinsulated panel in accordance with the invention having a variety offeatures.

FIG. 3 is another broken perspective view of a composite structuralinsulated panel in accordance with the invention with a portion of oneouter surface removed for clarity.

FIG. 4 is a side sectional schematic of one application of the inventionas a bridge substructure retrofit.

FIG. 5 is a side perspective schematic of the formation of the bridgesubstructure retrofit of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION AND DRAWINGS

Description of the invention will now be given with reference to FIGS.1-5. It should be understood that these figures are exemplary in natureand in no way serve to limit the scope of the invention which is definedby the claims appearing hereinbelow.

A simplified schematic of a structural panel 10 in accordance with theinvention is shown in sectional view in FIG. 1. Panel 10 includes a foamcore 12 sandwiched between two fibrous layers 14 and 16. The fibrouslayers are impregnated with an epoxy in which nano-particles aresuspended; the epoxy causes fibrous layers 14 and 16 to adhere to foamcore 12, while the suspended nano-particles add strength or otherproperties to the composite. Because the epoxy and the foam chemicallyreact and form a well-bonded region 15 (see FIG. 3), there is little orno change of delamination of the fibrous layers from the foam core. Forexample, carbon nano-particles suspended in the impregnated epoxyprovide significant increases in strength, so much so that cheaperfibrous layers such as fiberglass or polyester can be used and yet stillachieve similar strength properties as using much more expensive wovencarbon fiber mats.

External layer 20 is adapted to face the exterior of a building and mayinclude conventional exterior masonry such as stucco, brickface, or thelike. External layer 20 may be applied directly to the outer surface offibrous layer 14 after the epoxy has completely cured (see below).Similarly, internal layer 22 is adapted to face the interior of abuilding and may include conventional interior surfacing material suchas drywall, wallpaper, wood paneling, etc.

Core insert element 30 is disposed inside foam core 12 and representsany of a number of different integratable conventional buildingstructures that are typically used in construction. FIG. 2 depictsstructural panel 10 with parts of its internal layer 22 in brokenperspective and with its foam core removed for clarity. In FIG. 2, anumber of different such exemplary core insert elements 30 are shown,for example: stiffening spines 31; water piping 32 and water supplynipples 33; electrical conduit 34 and light switch 35; electrical outlet36; junction box 37 and conduit connector 38; and the like.Additionally, perimetric structural elements such as sill plate 40 (alsoshown in FIG. 1), header fill 42, top plate 44, and similar elements maybe included in or on panel 10. It is contemplated that the inventivepanel may include any and all of these type of elements, and othersimilar elements not shown, but need not include even one of them tofall within the scope of the invention.

The manufacture of a structural panel in accordance with the inventionis as follows. For the preferred structural panel, carbon nano-particlesare used. The carbon nano-particles in the proper concentration (to bediscussed below) are suspended in a liquid epoxy. Specifically, carbonnano-particles are preferably added to the hardener portion of theepoxy, as it is thinner, and the nano-particles are more readily evenlysuspended and dispersed in the hardener. Once the nano-particles areevenly distributed through the hardener, thehardener-with-nano-particles suspension is mixed with the resin. This isadvantageous for the further mixing of the nano-particles throughout theepoxy mixture, as hardener molecules seek out resin molecules and helpdistribute the carbon nano-particles throughout the epoxy. As a result,there is little or no clumping of the carbon nano-particles in themixture, and an expensive mixing device is not needed.

The preferred weight ratio of carbon nano-particles to epoxy ranges from1:200 to as much as 1:2, though more preferably the range is 1:100 to1:10. Depending on the properties desired of the resultant panel, theratio can be adjusted. Using less than 0.5 g of carbon nano-particlesfor every 100 g of epoxy yields very little strength benefit. Using morethan 50 g of carbon nano-particles for every 100 g of epoxy yields apasty, brittle mixture unsuitable for use.

In any event, the fibrous layer is wet with the nano-particle and epoxysolution, forming the composite. The composite is then cured using themethod described in U.S. Pat. No. 6,117,376 or by use of an autoclave.Specifically, as mentioned above, a mold is created and fibrous layersof one or more forms line the inside surfaces of the mold halves. In theinventor's previous patent, the fibrous layers were preferably carbonfiber reinforced at the least, if not entirely made from carbon fiber.As a result, the product, although of excellent strength and durability,is too costly to be practical on a mass-produced scale.

Instead, the fibrous layers can be made from much less expensivematerials, such as polyester, nylon, or fiberglass. In a preferredembodiment, if polyester is used, it should be in the form of a wovenpolyester mat. If fiberglass is used, it can be either a chopped-strandfiberglass mat or a continuous filament fiberglass mat (or both). Thefibrous layer may include two fiberglass mats, each of the fiberglassmats being either a chopped-strand fiberglass mat or a continuousfilament fiberglass mat. Preferably, one layer of fiberglass and onelayer of either polyester or nylon as a backing layer are used. Whateveris used, the fibrous layers are wetted with thenano-particle-impregnated heat-curable epoxy, and the mold halves areclosed together to form an interior cavity having an opening. Exothermicfoam is introduced into the mold cavity in an amount so as to overfillthe mold. The foaming material expands and heats up, causing theheat-curable epoxy to cure. Since an abundance of foam is provided,excess foam spills out of the opening in the cavity, but because thefoam expands so rapidly, the pressure inside the mold causes the fibrouslayers to be pressed flat against the inner surfaces of the mold. Inaddition, the epoxy cures (from the heat generated by the exothermicreaction of the foam) and causes the fibrous layers to stick securely tothe internal foam. The resulting composite is exceptionally strong andcan be created in substantially any shape. Also, as mentioned above,various other construction elements may be molded within the foam.

An added benefit of the carbon nano-particles is that their presencecauses heat-curable epoxy to cure faster, up to 50% faster. For example,5-minute epoxy in which carbon nano-particles have been suspended hasbeen observed to cure in 2-3 minutes, depending on the ratio ofnano-particles to epoxy used.

Once cured, the composite structure has various applications and isideally suited for structural components that require low weight, highshear, high load, and high tensile strengths. The preferred embodimentfocuses on building applications and centers around a product called acomposite structural insulated panel (CSIP). The CSIP can be used forwall, floor, and roof panel systems in residential and non-residentialbuildings where low-cost, highly insulated structural components areadvantageous. A CSIP includes a polyurethane core and has outer skinscomprised of low-cost chop-strand fiberglass mat and polyester wovenmat. The skins are bonded to the polyurethane by a nano-particle andepoxy solution as described above, forming a composite suitable forbuilding. The CSIP can be made with any style edge to accommodatedifferent designs for different applications. Because the CSIP can bemolded into essentially any shape, an ordinary wood 2×4 may be used as aspline, or the panels can be simply glued together if formed with atongue and groove system.

The CSIP is extremely strong yet lightweight and compares favorably toboth wood SIPs and concrete blocks. The CSIP exceeds the InternationalCode Council's Acceptance Criteria for Sandwich Panels (ICC-ES AC04) andAcceptance Criteria for Shear Wall Assemblies Consisting of WoodStructural Panel Sheeting Attached to Cold-Formed Steel Framing withPneumatic or Gas-Power-Driven Fasteners (ICC-ES AC230). The framingcosts for a CSIP are about the same as those of a wood SIP and about 40%of that of concrete block, yet the shipping costs (owing to itsextremely light weight construction and the ability for it to be made onsite, if need be) are lower than those of either wood frame, wood SIPs,or concrete. The CSIP is extremely high in corrosion and insectresistance (like concrete, unlike wood frame or wood SIps), yet it isvery easy to work with and easy to assemble (like wood SIPs, unlikeconcrete). The CSIP also has superior sound insulation to either woodSIPs or concrete block. For extra sound insulation, carbon nano-tubescan be added to both the epoxy during the mixing process and the foamprior to injection into the mold. In addition, as alluded to above, theexterior side of a CSIP is ready to accept stucco or other similarmasonry, while the interior side may be provided with an integraldrywall finish; none of wood frame, wood SIPs, nor concrete blockexhibits such design flexibility.

Since the carbon nano-particles retain heat well, this property alsomakes the CSIP manufactured by the inventive method an excellentinsulator; they typically have a wall R-Value of R-30 and a roof R-Valueof R-60. This compares very favorably to a typical wood SIP (wallR-Value of R-15, roof R-Value of R 30) or concrete block (wall R-Valueof R-19, roof R-Value of R-30). As such, the CSIP makes an excellentroofing tile as well as a structural wall. One preferred formulation ofroofing tile utilizes 10 g of carbon nano-particles for every 100 g ofepoxy; this formulation provides the thus-made roofing tile withexcellent thermal properties. Roof panel CSIPs reduce the heat islandeffect of a building because they have a high thermal emittance andrelease stored heat.

Since the CSIP is a structural element, it need not be limited to newconstruction applications; it can also be used to retrofit existinginfrastructure far more quickly, inexpensively, and practically thanrebuilding or using a traditional steel retrofit. FIGS. 4 and 5 depictone such retrofit, of bridge substructure support columns. In FIG. 4, anexemplary roadway 100 is supported underneath by columns 110. As happenseventually with all support columns subject to wear and tear andexposure to the elements, cracks 112 have begun to develop. Rather thanreplace the entire bridge or even an entire column with another column,or even the conventional retrofit fix of cladding the column in a steeland concrete jacket, a wrap 120 of the inventive structural material maybe employed instead.

FIG. 5 shows a typical cylindrical bridge column 110. For this type ofapplication, it is preferred to apply a coating of carbon nano-particleimpregnated epoxy directly to column 110, and then affix at least onefibrous layer to the epoxy-coated column. It is preferred that enoughepoxy be applied to the column that it soaks through and fully wets thefibrous layer. A hollow cylindrical mold 140 made of at least two moldsections 142 and 144 is provided. At least one fibrous layer is placedin each of mold sections 142 and 144 and wetted withnano-particle-impregnated epoxy. Mold 140 is closed around column 110and is dimensioned so that there is clearance between the fibrous layerplaced on mold 140 and the fibrous layer placed around column 110. Aswith the flat panel, exothermic foaming material is introduced into thegap between the mold and the column. The foam expands and releases heat,which cures the epoxy and causes the entire retrofit structure toharden. The result is a retrofit that may be performed and installed insitu without requiring the closing of the roadway above as with aconventional steel jacket retrofit (so long as the damage to thesubstructure had not already progressed farther than would allow trafficabove). Additionally, the inventive composite nano-particle retrofit ismuch lighter than the steel jacket retrofit and takes about half thetime. Finally, the inventive retrofit does not require the replacementof the column footing, which is required with a steel jacket retrofit.

The substructure wrap may entirely encase a support column, however itneed not do so. The majority of damage to and failures of bridge supportcolumns occur in the bottom third of the column. As such, as shown inFIG. 4, the foam core of the wrap need only extend the lower 80% of thelength of the column, leaving the upper 20% covered only in fibrouslayers with nano-particle-impregnated epoxy.

The preferred size range for carbon nano-particles is from 10 nm to asmuch as 200 nm in diameter. Carbon nano-particles suitable for use maybe as-grown, pyrolytically stripped (of surface polyaromatichydrocarbons), or heat treated. As-grown nano-particles are the leastexpensive and help keep the overall cost of a CSIP down. Alternatively,carbon nano-fibers of 70-200 nm wide and 50-100 microns long may beused. As a substitute for carbon, silicon carbide and titanium carbidenano-particles may be employed. All of the above-mentionednano-particles may be obtained from American Elements of Los Angeles,Calif.; the carbon nano-particles and nano-fibers are suitably producedby Applied Sciences, Inc. of Cedarville, Ohio under the Pyrograph name;appropriate carbon nano-particles are also available from Asbury Carbonsof Asbury, N.J.

As mentioned above, the invention allows the replacement of expensivewoven carbon fiber in construction with much less expensive fiberglassor polyester layers (or both). In one embodiment of the inventivestructural panel, a bulk laminate polyester layer is used in conjunctionwith either a chopped strand or continuous filament fiberglass layer.One preferred bulk laminate polyester is made by Fibre Glast ofBrookville, Ohio, model no. 2258-A1 (either in 2 mm or 4 mm). Onepreferred continuous filament fiberglass layer is produced myOwens-Corning as continuous filament mat no. 44986-NAM (preferably the 8oz. mat). As an alternative to polyester, nylon may be used.

Thousands of epoxy resins are available for use in forming the CSIP,though some are more advantageously used in certain applications. Forexample, marine resins are generally excellent for use in outdoorapplications in high humidity environments or for the retrofit of bridgesupport columns which are in or near bodies of water. Other types ofepoxy resins that may be employed are room-temperature curable resins(which may nevertheless be accelerated in their curing by the additionof heat from the exothermic foam); fire resistant/retardant resins, fordwellings and commercial space; and high impact resistant resins, forballistic and armor applications. One epoxy system that the inventor hasused with success is the EL-319 series made by CASS Polymers of MadisonHeights, Mich. Using the EL-319 resin with either the EL-319 hardener orthe EL-319-1 longer work life hardener yields a structural panel that isextremely flame retardant (approximately four hours) and otherwiseversatile.

The preferred type of exothermic foam is polyurethane, specificallyclosed cell foam. Open cell foam is typically not strong enough toprovide adequate support for a structural panel. Foam density is alsoimportant to the strength and insulation value of the structural panel,however the required density varies from application to application.Closed cell 4-lb density foam has the best R-Value per strength ratioand is best suited for most CSIP applications. Closed cell 6-lb densityfoam has higher strength than 4-lb foam but a lower R-Value; 6-lb foammay be more useful in panels having a number of core insert elements 30(see, e.g., FIGS. 1-2); the more core insert elements a panel has, theweaker the panel becomes. Higher density foam (e.g., 8-lb or 16-lb) hasgreater strength but lower insulative properties. Foam with lowerdensity than 4-lb foam (e.g., 2-lb foam) may be extremely lightweightand an excellent insulator but will not have enough structural strengthfor use with most CSIP applications. In any event, additives may beadded to the foam for purposes of strength, flame or insect resistance,sound insulation, or other reasons. For example, the addition of carbonnano-tubes to the foam of a CSIP greatly increases its sound insulationproperties.

One preferred embodiment of the CSIP panel uses one layer of E-typechopped strand fiberglass mat with one layer of bulk laminate polyesteras a backing pad together as the fibrous layer. The fibrous layer iswetted with fire resistant epoxy (e.g., the EL-319 series of CASSPolymers) which has been impregnated with carbon nano-particles in aratio of 2 g of carbon nano-particles to 100 g of epoxy. Either 4-lb ora soy-based 6-lb polyurethane foam is used as the core.

In another embodiment, ceramic nano-particles may be substituted forand/or added to the carbon nano-particles to provide or augment theballistic shielding protection a panel may offer. Traditional ballisticplates are formed from solid ceramic such as boron carbide (B₄C),alumina (Al₂SO₃), or silicon carbide (SiC). However, solid ceramicplates of B₄C and similar materials are extremely expensive. Instead, inaccordance with the invention, fibrous layers such as polyester orfiberglass are wetted with ceramic nano-particle-impregnated epoxy andheat cured. The foam core is not necessary for ballistic plates orpanels. Panels made in this manner are nearly as strong as solid ceramicplates of the same material but are far less expensive.

Smaller panels made by the above-described method may also be used asballistic insert plates or packs for body armor, vehicular armor, andother ballistic applications. Larger panels may be provided as the coreinsert elements 30 (see FIG. 1) of a regular CSIP and provide ballisticprotection features to the structural panel. Ceramic nano-particles bestsuited for this application include B₄C and alumina.

The invention is not limited to the above description. For example,while carbon nano-particles have been described for use in structuralapplications and ceramic nano-particles have been described for use inballistic applications, a mix of both carbon and ceramic nano-particlesmay also be employed. Further, additional features may be added to theCSIP via the addition of other types of nano-particles. For example,EMI/RFI shielding can be provided for military and industrialapplications in which eavesdropping protection is required. Aluminum,copper, nickel, and silver nano-particles can be used for this purposealong with either or both carbon and/or ceramic nano-particles.

Having described certain embodiments of the invention, it should beunderstood that the invention is not limited to the above description orthe attached exemplary drawings. Rather, the scope of the invention isdefined by the claims appearing hereinbelow and any equivalents thereofas would be appreciated by one of ordinary skill in the art.

1. A composite structural element comprising: a polymer foam core; atleast one fibrous layer adhered to said polymer foam core by epoxy; andnano-particles suspended in said epoxy while said epoxy is still inliquid form.
 2. A composite structural element according to claim 1,said polymer foam core comprising an exothermic foam, wherein heatgenerated by said exothermic foam cures said epoxy, thereby causing saidfibrous layer to be adhered to said foam core.
 3. A composite structuralelement according to claim 1, further comprising a bulk laminate layerdisposed between said polymer foam core and said fibrous layer.
 4. Acomposite structural element according to claim 1, said polymer foamcore comprising a polyurethane.
 5. A composite structural elementaccording to claim 1, said nano-particles comprising carbonnano-particles.
 6. A composite structural element according to claim 1,said nano-particles having a diameter of approximately 10-200 nm.
 7. Acomposite structural element according to claim 5, said carbonnano-particles comprising nano-fibers of approximately 60-200 nm indiameter and approximately 30-100 μm in length.
 8. A compositestructural element according to claim 5, wherein a weight ratio of saidcarbon nano-particles to said epoxy is at least 1:200 and no more than1:2.
 9. A composite structural element according to claim 5, wherein aweight ratio of said carbon nano-particles to said epoxy is at least1:100 and no more than 1:10.
 10. A composite structural elementaccording to claim 1, said nano-particles comprising at least one ofcarbon, a ceramic, tungsten, a carbide, titanium, zircon, aluminum,silver, or boron.
 11. A composite structural element according to claim1, said fibrous layer comprising: a woven polyester mat; and afiberglass mat.
 12. A composite structural element according to claim11, said fiberglass mat comprising at least one of a chopped-strandfiberglass mat or a continuous filament fiberglass mat.
 13. A compositestructural element according to claim 1, said fibrous layer comprising apolyester mat.
 14. A composite structural element according to claim 1,said fibrous layer comprising at least one of a chopped-strandfiberglass mat or a continuous filament fiberglass mat.
 15. A compositestructural element according to claim 1, said fibrous layer comprisingtwo fiberglass mats, each of said fiberglass mats being either achopped-strand fiberglass mat or a continuous filament fiberglass mat.16. A composite structural element according to claim 5, wherein aweight ratio of said carbon nano-particles to said epoxy issubstantially 1:10, and said structural element comprises a thermallyconductive roofing panel.
 17. A composite structural element accordingto claim 5, wherein said structural element comprises a structuralinsulated panel.
 18. A composite structural element according to claim17, said at least one fibrous layer further comprising two fibrouslayers on opposite sides of said foam core, both of said two fibrouslayers adhered to said foam core by said nano-particle-impregnatedheat-curable epoxy.
 19. A composite structural element according toclaim 18, further comprising an integrated dry wall finish applied to afirst outer surface of one of said two fibrous layers.
 20. A compositestructural element according to claim 19, further comprising exteriormasonry applied to a second outer surface of the other of said twofibrous layers.
 21. A composite structural element according to claim 2,said nano-particles having a high thermal conductivity relative to saidepoxy, wherein the addition of said nano-particles reduces the curingtime of said epoxy.
 22. A composite structural element according toclaim 1, said nano-particles comprising at least one ceramic and saidstructural element comprising a ballistic shielding element.
 23. Acomposite structural element according to claim 22, said ceramicnano-particles comprising at least one of boron carbide and alumina. 24.A composite structural element according to claim 2, said nano-particlescomprising carbon and said structural element comprising a structuralwrap formed around an existing load-bearing element.
 25. A compositestructural element according to claim 24, wherein said structural wrapcomprises a bridge substructure wrap and the existing load-bearingelement is a bridge support column.
 26. A composite structural elementaccording to claim 25, wherein said wrap substantially encompasses allof the bridge support column and said foam core extends along at leastapproximately 80% of the length of the column.
 27. A method of making acomposite structural element comprising the steps of: mixingnano-particles into an epoxy; providing at least one fibrous layer andwetting the fibrous layer with the nano-particle-impregnated epoxy; andintroducing a heat source to cure said epoxy.
 28. A method of making acomposite structural element according to claim 27, saidheat-introducing step further comprising the steps of providing acatalyzed foamable exothermic material in thermal communication with thewetted fibrous layer.
 29. A method of making a composite structuralelement according to claim 28, further comprising the steps of:providing two wetted nano-particle-impregnated fibrous layers in a mold;introducing the exothermic foam in direct contact with and in betweenthe fibrous layers, thereby curing the epoxy and bonding the fibrouslayers to the foam to form a foam core sandwiched between the twofibrous layers.
 30. A method of making a composite structural elementaccording to claim 28, further comprising the steps of: providing twofibrous layers in a mold; placing a flexible receptacle having a wall,at least one opening, and an interior between the two fibrous layers;introducing the exothermic foam into the interior of the receptacle viathe opening; and curing the epoxy with heat released from the exothermicfoam that passes though the wall of the receptacle.
 31. A method ofmaking a composite structural element according to claim 27, said mixingstep further comprising the steps of: adding the nano-particles to thehardener of the epoxy; and then mixing the nano-particle-impregnatedhardener with a resin.
 32. A method of making a composite structuralelement according to claim 27, wherein the nano-particles have adiameter of approximately 10-200 nm.
 33. A method of making a compositestructural element according to claim 27, wherein said nano-particlemixing step is performed so that a weight ratio of the nano-particles tothe epoxy is at least 1:100 and no more than 1:2.
 34. A method of makinga composite structural element according to claim 27, wherein thenano-particles comprise carbon and said nano-particle mixing step isperformed so that a weight ratio of said carbon nano-particles to saidepoxy is at least 1:100 and no more than 1:10.
 35. A method of making acomposite structural element according to claim 27, wherein thenano-particles added during said nano-particle mixing step include atleast one of carbon, a ceramic, tungsten, a carbide, titanium, zircon,aluminum, silver, or boron.
 36. A method of making a compositestructural element according to claim 28, further comprising the stepsof: placing the wetted nano-particle-impregnated fibrous layer beforecuring around an existing load-bearing element; placing a mold and asecond wetted nano-particle-impregnated fibrous layer around the firstfibrous layer and spaced apart from the first fibrous layer; andperforming said exothermic foam providing step by introducing theexothermic foam into the mold between the two wetted fibrous layers. 37.A method of making a composite structural element according to claim 36,wherein said foam introducing step further comprising the step ofproviding the foam for at least approximately 80% of the length of theexisting load-bearing element.